A chemical-mechanical planarization (CMP) procedure is widely used in a semiconductor manufacturing process for planarizing a wafer. The apparatus and the operating cycle for a CMP process is briefly described with reference to FIG. 1 which is a schematic cross-sectional view of a conventional CMP apparatus. In the CMP apparatus, a platen 6 is rotatably mounted, and a polishing pad 4 is secured on and rotated with the platen 6. The back surface of a wafer 8 to be polished is held by a carrier 7 and parallelly placed on the platen 6 so that the front surface of the wafer 8 is in contact with the polishing pad 4. The carrier 7 also rotates during the CMP process in a direction the same as the platen 6. During the CMP process, polishing slurry 9 is continuously provided onto the polishing pad 4 for facilitating to polish the wafer 8. The slurry 9 is supplied through a pipeline 5 arranged above the platen 6. After a CMP cycle in which a wafer or several wafers is or are simultaneously planarized is completed, the polishing pad 4 is subjected to a conditioning cycle to restore its proper function. Therefore, in brief, the purpose for polishing a wafer can be achieved by gradually removing bumps on the surface of the wafer via a chemical reaction conducted by the polishing slurry and a mechanical force applied to the wafer through the operations of the above devices.
The control of the CMP process for precisely planarizing a semiconductor wafer, however, is relatively complicated because of a great number of variations in the process. The condition of the polishing pad is one of the variations which are considered uneasy to be controlled. In other words, with the increasing abrasion in the CMP cycles and the conditioning cycles, the condition of the polishing pad is gradually deteriorated, and the pad should be changed when the pad surface is lowered to an unacceptable value. Therefore, it is important to monitor the condition of the polishing pad frequently.
Monitoring methods used in a CMP process is various. For example, Sandhu et al, U.S. Pat. No. 5,036,015, monitors a planar endpoint by measuring the current change in the driving motor of the carrier. Yu et al, U.S. Pat. Nos. 5,222,329 and 5,240,552, monitor an endpoint and thickness of films formed on a wafer by analyzing acoustical waves and reflected acoustical waves from the wafer. Yano et al, U.S. Pat. No. 5,483,568, monitors a polishing rate by detecting the intensity of the X-ray fluorescence resulting from an X-ray beam illuminating the polishing pad.
Meikle et al., U.S. Pat. Nos. 5,609,718, 5,655,951 and 5,801,066, disclose a method and an apparatus for measuring a change in the thickness of the polishing pad by using a laser beam detector. This method for monitoring the thickness change of the polishing pad is conducted by measuring the thickness change at a plurality of specified points after the pad is conditioned. In other words, the thickness of discontinuous points on the surface of the polishing pad is measured. As known to those skilled in the art, however, thickness measurement may be interfered by the polishing slurry so that there may be some irreliable data present. For such a sampling way, it is difficult to determine which data points are not reliable and should be ignored. On the other hand, the thickness measurement of the above method is conducted after the pad is conditioned and cannot be performed during the CMP cycle so that a thickness-measuring step is required in addition to the CMP cycle and the conditioning cycle, and the purpose for on-line monitoring of the polishing pad cannot be achieved.